The present invention relates to a developing unit for developing an electrostatic latent image or a magnetic latent image using a two-component developer in which magnetic carrier particles and toner particles are mixed, in an electrophotographic copier, and the like.
Conventionally, in an electrophotographic copier, a magnetic brush development type developing unit using a two-component developer, is used. This developing unit has a cylindrical developing sleeve and a magnet roller composed of a magnet body having a plurality of magnetic poles therein, and which is rotatably supported. Magnetic carriers, to which toner particles are adhered, are held on the surface of the developing sleeve, and conveyed to a developing area for development. This developing unit has the following features: control of triboelectricity of the toner particle is relatively easy; coagulation of toner particles rarely occurs; bristling of the magnetic brush is good; the frictional property of the surface of an image carrier is superior; and when the developing unit is also operated for cleaning, the cleaning effect is very satisfactory. Although, in this type of developing unit, control of the amount of toner with respect to that of the carrier particles is necessary, this type of developing unit is used very often. However, in the developing method in which this magnetic brush is rubbed on the surface of the image carrier for development, conventionally, the developer composed of the magnetic carrier particles having an average diameter in multiples of ten .mu.m through multiples of hundred .mu.m and the non-magnetic carrier having an average diameter of about 10 .mu.m is used. Since the diameter of toner particles and carrier particles is large, problems exist in which a high quality image for reproducing fine lines or dots, or the difference of densities, can hardly be obtained. In order to obtain the high image quality, conventionally, many technologies such as, for example, resin coating of the carrier particle, and improvements in the magnet body in the developer conveyance body, have been used. However, stable and satisfactory images can not yet be obtained. Accordingly, in order to obtain higher quality images, the following has been considered that it is necessary to make the diameter of toner and carrier particles smaller. However, when the toner particles are reduced to an average particle size of not more than 20 .mu.m, particularly not more than 10 .mu.m, the following difficulties occur: 1 the influence of Van der Waals forces appear relative to the Coulomb force at the time of development; the adhesive force between the image forming body and toner becomes strong; so-called fogging in which toner particles adhere to a background portion of the image, occurs; and it is difficult to prevent fogging even when a DC bias voltage is impressed upon a developer conveyance body. 2 Triboelectricity control of toner particles becomes difficult, and coagulation easily occurs. On the other hand, as carrier particles are made finer, 3 carrier particles adhere to an electrostatic image portion of the image carrier. As a reason for this phenomenon, the following can be considered: the force of magnetic bias is lowered, and carrier and toner particles adhere to the image carrier side. When the bias voltage becomes larger, carrier particles also adhere to the background portion of the image. When particles are made finer, there are problems in which the above-described side effect becomes more conspicuous, and a sharp image can not be obtained. Accordingly, when toner and carrier particles are made finer, difficulties occur in the actual use of the finer particles.
In order to solve the above-described problems, the following methods have been proposed: 1 a method in which the deevloper is conveyed to a developing area in such a manner that the developer is not in contact with the image forming body, toner in the developer is scattered by an oscillation electric field, and the latent image is developed (Japanese Patent Publication Open to Public Inspection No. 222847/1984); and 2 a method in which, in a non-contact developing method, a horizontal magnetic field is formed on the developing area, a smoothing member is provided between the central portion of the developer area and a regulation member for regulating the layer thickness of the developer layer, and a DC bias voltage having a reverse polarity to the charging polarity of toner particles is impressed upon the smoothing member (Japanese Patent Publication Open to Public Inspection No. 94368/1989).
Further, 3 a toner cloud developing method using a plate-shaped electrode body (Japanese Patent Publication Open to Public Inspection No. 131879/1991) has been disclosed.
However, the above-described method 1, the following problems occur: when the average particle size of toner particles is not more than 10 .mu.m, since the influence of the Van der Waals forces becoms large as previously described, the adhesive force between carrier and toner is increased, so that the developability is lowered extremely.
In the above-described method 2, there are the following problems: bristles of the magnetic brush collapse due to the horizontal magnetic field, so that the developer layer is made denser; accordingly, toner is barely extracted by the smoothing member; and especially, when small-sized toner particles having an average particle size of not more than 10 .mu.m are used, the developability is extremely lowered. Further, since a DC voltage having the reverse polarity to that of toner is impressed upon the smoothing member, toner particles are accumulated during the progress of the devloping operation and the image is stained.
Further, in the above-described method 3, there is a problem in which: since the electrode body is the plate-shaped member, the toner cloud is generated at the contact position of the electrode body or even on the upstream portion of the closest-position of the electrode body, and the amount of developer conveyed is lowered, so that development can not be correctly carried out.
The first object of the present invention is to provide a developing unit by which an image is not stained, and stable and high developability can be obtained even when smaller toner particles and carrier particles are used, by the method in which where toner particles in the developer are made to fly by the oscillation electric field after a two-component developer has been smoothed by a plate-shaped member.
The second object of the present invention is to form a high density and uniform developer layer on the developer conveyance body when a plate-shaped elastic body is located in such a manner that it is used as a smoothing member, and is pressed on the developer on the developer conveyance body. This technology to attain the second object can be used for a general contact type development method, and can be effectively used, especially for the non-contact type development method.